Because a lamp series, such as a Christmas lamp series, has a good decorating effect, it is well adored by the people. The lamp series is an electric article such that its structure, safety, and performance need to be taken into account. Two kinds of lamp series in prior art shown in FIGS. 1 and 2 have their drawbacks. As shown in FIG. 1, a lamp series is a power cable with a plurality of bulb sets installed thereon. Each bulb set comprises a light bulb 10, a soft core 12, and a soft head 14. After the light bulb 10 is inserted in the soft core 12, conducting threads 16 are respectively bent upwards at two sides of the soft core 12. The soft core 12 is then lapped in the soft head 14 such that the soft core 12 is electrically connected to conducting plates 18 in the soft head 14. But the soft core 12 of the lamp series must be lapped exactly in the soft head 14. If the soft head 14 is too large, the conducting threads 16 will not contact well with the conducting plates 18 such that the conduction is not good and the soft core 12 and the light bulb 10 thereon will easily become loose so as to shed. If the soft head 14 is too small, it is difficult to lap the soft core 12 in the soft head 14. The assembly will be difficult and cumbersome.
As shown in FIG. 2, another lamp series in prior art is a power cable with a plurality of copper caps 20 installed thereon. An inner thread is formed in the copper cap 20. A copper head 26 having an outer thread is formed at the bottom of a light bulb 24 such that the copper head 26 of the light bulb 24 can be screwed in the copper cap 20 to achieve electric connection. Although assembly of this kind of lamp series is convenient, the metallic material of the copper head and the copper cap makes its cost high. Moreover, metallic material has no waterproof effect.